Hepatitis C virus (HCV) is a major causative agent of acute and chronic hepatitis worldwide. It is estimated that there are 200 million chronically HCV-infected individuals worldwide, 4 million of whom reside in the United States. The foremost source of infection is through parenteral routes including blood transfusions or IV drug use. Despite the high degree of safety associated with current blood banking procedures, the rate of infection continues to increase, presumably due to IV drug use and other forms of exposure.
According to data from the Third National Health and Nutrition Examination Survey (NHANES III), approximately 70% of the patients with HCV infections in the United States will become chronically infected. A significant proportion of chronically infected individuals will suffer a serious sequelae of chronic HCV infection including progression to cirrhosis, hepatic decompensation, liver transplant, hepatocellular carcinoma, and death. Retrospective long term follow-up studies on patients chronically infected with HCV estimate the proportion who will progress to cirrhosis at approximately 20% to 50% with follow-up times ranging from 10 to 29 years (1-4). Prospective long term follow-up studies on patients chronically infected with HCV after post-transfusion exposure estimate the proportion who will progress to cirrhosis at approximately 10% to 15% with relatively short follow-up times ranging from 8 to 16 years (5-8). Of those patients who develop cirrhosis secondary to viral infection it is predicted that approximately 1% to 3%, will develop hepatocellular carcinoma annually with an approximate annual mortality rate of 2% to 6% (9-10). An epidemiologic model utilizing NHANES III seroprevalance data and age-specific incidence rates estimates a peak in U.S. population risk for progression to cirrhosis and related complications by 2015, foretelling of a worsening unmet medical need in the near future (11). Interruption of the chronic viral infection using interferon based regimens has been shown in several large series to favorably alter the rates of progression to cirrhosis, hepatocellular, and death (12-14). However, sustained virologic response rates for the treatment of genotype 1 chronic hepatitis C, the predominant genotype found in the U.S., are only approximately 50% with pegylated interferon-α regimens containing ribavirin. Additionally, interferon plus ribavirin based regimens also have significant safety problems including depression, suicidal ideation, flu-like symptoms, and neutropenia. Treatment options are currently limited for partial responders, relapsers, and non-responders to interferon based therapy.
HCV is a member of the Flaviviridae family of enveloped, positive-sense RNA viruses. It has a genome of approximately 9600 nucleotides that is translated upon cell entry into a polyprotein of roughly 3000 amino acids. Three structural and seven non-structural proteins are generated co- and post-translationally by cellular and HCV-derived proteases (Table 1). While the roles of some of the viral proteins have yet to be clearly defined, a number of them, such as the HCV structural Core protein, the E1 and E2 surface glycoproteins, the non-structural NS2 and NS3 proteases, and the NS5B RNA-dependent RNA polymerase are known to perform essential functions in the HCV life cycle. Based on genetic heterogeneity of the viral genomes isolated so far, HCV has 6 major genotypes and more than 100 subtypes.
Genotypes 1a, 1b and 2 are found predominantly in North America and Europe, while in South America, HCV genotypes 1a, 1b, and 3 are prevalent. Genotypes 4, 5 and 6 are observed throughout the rest of the world (19). Despite the geographic predominance of certain HCV genotypes, most genotypes have been identified all over the world due to increased population movement. The different HCV genotypes vary in terms of their response to the currently recommended interferon/ribavirin therapy. In particular, ˜50% of patients infected with HCV genotype 1 remain refractory to the current treatment regimen (19). Further, response rates to interferon alpha among African-American patients are lower than those of Caucasian descent. These data suggest the need for alternative treatments that ideally augment the individual's pre-existing cellular immune response.
TABLE 1HCV genes and gene products% homology between HCVGeneFunctiongenotypes 1a and 1bCoreNucleocapsid core protein98.4E1Envelope glycoprotein81.8E2Envelope glycoprotein79.9P7Ion channel81.0NS2metalloprotease80.1NS3protease/helicase92.1NS4aNS3 protease co-factor91.1NS4bUnknown82.4NS5aUnknown77.7NS5bRNA-dependent RNA polymerase87.5The HCV protein sequences were obtained from the National Center for Biotechnology Information under Accession No. AF011753 (gi: 2327074). The Align program from the Genestream Bioinformatics website (Institut de Génétique Humaine, 141 rue de la Cardonille, Montpellier France) was used to compare the amino acid sequences of the HCV proteins derived from strain 1a and 1b.
Numerous studies suggest that viral replication, the level of viremia and progression to the chronic state in HCV-infected individuals are influenced directly and indirectly by HCV-specific cellular immunity mediated by CD4+ helper (TH) and CD8+ cytotoxic T lymphocytes (CTLs), and directed against both structural and non-structural viral proteins including Core and NS3 (15). The lack of effective immunity in persons with chronic HCV infection is further implied by the occurrence of superinfection with other genotypes of HCV. As the robustness and breadth of cellular immune responses have been suggested to influence the natural course of HCV infection, the development of immunotherapeutic products that stimulate T cell immune responses in virally exposed individuals is of major importance.
Studies of humans and chimpanzees have revealed that HCV can replicate for weeks before the onset of CD4+ and CD8+ T cell responses in blood and liver. Moreover, there may be a delay in the acquisition of function by CD8+ (and perhaps CD4+) T cells even after their expansion in blood (15). The appearance of functional CD8+ T cells is kinetically associated with control of viremia and, at least in some cases, with an elevation in serum transaminases, suggesting that liver damage during acute hepatitis C is immunopathological. At highest risk of persistent HCV infection are those individuals who fail to generate a detectable virus-specific T lymphocyte response in the blood, liver, or both. Perhaps most importantly, generation of a cellular immune response does not necessarily ensure that the infection will be permanently controlled. CD4+ and CD8+ T cell responses must be sustained for weeks or months beyond the point of apparent control of virus replication to prevent relapse and establishment of a persistent infection.
CD4+ T cells play an essential role in anti-HCV immunity by providing help for activating and sustaining CD8+ T cell responses. Protective CD4+ T cells appear to predominantly recognize epitopes in Core, NS3, NS4 and NS5 proteins although responses against the other HCV gene products have also been reported (20-21). In addition to the help that CD4+ T cells provide to CD8+ T cells, it also appears critical that they produce gamma interferon and other pro-inflammatory TH1-, as opposed to, TH2-type cytokines. Equally important for control of chronic infection is the establishment of HCV-specific memory CD4+ T cells (20 & 22).
The finding that CD4+ and CD8+ T cell responses are common to self-limited HCV infections suggests that they cooperate to bring about control of viremia. Memory CD4+ and CD8+ T cells primed during acute resolving hepatitis C infection provide long-term protection from virus persistence in chimpanzees and probably humans. Through antibody-mediated depletion of each memory T cell subset, the chimpanzee model has provided direct proof of the importance of CD8+ T cells in the control of acute hepatitis C and their dependence on CD4+ T cell help (24). In contrast to CD4+ T cells, both acute and memory CD8+ T cells appear to recognize all of the HCV proteins equally and, as with CD4+ T cells, it may be critical that they be capable of producing pro-inflammatory cytokines including gamma interferon (15).
The transition from acute to chronic HCV infection is associated with substantial loss of HCV-specific CD4+ T cells that do not appear to recover during the life of the host. CD8+ T cell activity is also impaired, as it is insufficient for resolution of infection.
A number of experimental approaches to immunotherapy in general have been investigated, including the use of DNA-, recombinant viral-, and autologous dendritic cell-based vaccine strategies. DNA vaccines are good at priming immune responses in humans but are poor at boosting. In contrast, recombinant viruses are good at boosting but suffer from the limitation of vector neutralization. Finally, dendritic cell-based vaccines are patient-specific and labor intensive. Therefore, there remains a need in the art for an effective immunotherapeutic approach against HCV.